Here are a few pointers to tell if a pin is a scrapper/counterfeit pin.

Scrappers are left overs from the pin making process while counterfeits are unauthorized pins that usually made from the same molds(not always) as real pins. As such scrappers might seem unfinished and most counterfeits are made with lesser materials.
Look out for a dull quality, authentic pins have a shiny enamel finish with a slight polish to the the metal.
Look at the edges of the pin; legitimate pins usually have perfectly smooth edges. If it doesn't, this could be a scrapper.
Pay attention to numbered pins. If they are legitimate, they will have the number on them(ex. 18/300). If it is an unauthorized pin, it will be blank(ex. /300).
Take a very careful look at the back for the Disney Disney Pin Trading logo. The logo will range from slightly cloudy to clearly illegible on a fake. Also look out for uneven boxes or logos in contrast to the pin's front, it could mean its a fake pin.
Newer store pins have a Mickey Head Shaped layering pressed onto the back of them. The backing should run off the edges, but countefeits usually display a noticible border instead of running off.
When buying pins online, beware of descriptions that seem be too good to be true. There are certain words and phrases the seller may use which 9 times out of 10 will make it known that it is a fake.

The phrases "all pins have the Disney Copyright" and "all pins are tradable in the park" are usually found on multi-lot scrapper/counterfeit pin sales. The words don't disclose authenticity nor where the seller got them, even fakes can have the disney copyright on them. The sad truth is that those are the minimal requirements of Cast Member trading which allows most unathorized pins to still be traded.
The seller may also point out the many positive reviews that they've recieved. See if they've recieved bad reviews, they might be looking to rip you off.

Ultimately, find out about past fake pins and follow-up on the trends. Use online sites as Pinpics or pin comparison threads to find out more information on unauthorized pins.

Be smart; do your research and you can go far and not get ripped off. If you do feel you have been ripped off, then you need to contact or return to the seller. Also, keep that these scrappers are 100% illegal.